jme: Improve tiny packets transmission performance on low frequency CPU

jme: Improve tiny packets transmission performance on low frequency CPU

Update TXCSR register a little bit often; mainly to improve timeliness of
packets transmission:
The TXCSR register is updated after certain amount of TX descriptors are
added to the hardware TX ring.  The default value of the amount of TX
descriptors are 16.  This value could be further tuned by per-device sysctl
node hw.jmeX.tx_wreg.

The default value improves tiny packets transmission performance w/ JMC250
on AMD970@2200Mhz (831Kpps -> 911Kpps), on AMD970@800Mhz (484Kpps -> 834Kpps)
and it does not increase CPU usage on AMD970@3500Mhz (CPU usage stays @26%,
JMC250 could only do 911Kpps).